1. Field of the Invention
The present invention relates to a light receiving member for electrophotography and a fabrication process thereof and, more particularly, to a light receiving member for electrophotography provided with a photoconductive layer comprising a non-monocrystal material the matrix of which is silicon atoms, and a fabrication process thereof.
2. Related Background Art
In the field of image formation the following characteristics are required of photoconductive materials for forming a light receiving layer of the light receiving member. That is, they should have high sensitivity, high SN ratios (photocurrent (Ip)/dark current (Id)), absorption spectra matching with spectral characteristics of electromagnetic waves injected into the light receiving member, quick optical response, and desired dark resistance, they should be harmless to the human body during operation, and so on. Especially, when the light receiving member is incorporated in an electrophotographic apparatus used as a business machine at office, the harmlessness during operation as described above is important.
Hydrogenated amorphous silicon is one of the photoconductive materials exhibiting excellent characteristics and an example of application thereof to the light receiving member for electrophotography is described in U.S. Pat. No. 4,265,991.
The light receiving member of this type is usually made by heating an electroconductive substrate at 50.degree. C. to 350.degree. C. and forming a photoconductive layer of amorphous silicon on this substrate by a film-forming process such as the vacuum vapor deposition process, the sputtering process, the ion plating process, the thermal CVD process, the optical CVD process, or the plasma CVD process. Among others, the fabrication process by the plasma enhanced CVD process is preferred and is used in practice. This plasma CVD process is a process for decomposing a source gas by high-frequency or microwave glow discharge and forming a deposited film of amorphous silicon on the photoconductive substrate.
Also proposed in U.S. Pat. No. 5,382,487 is the light receiving member for electrophotography in which the photoconductive layer of amorphous silicon containing halogen atoms is formed on the conductive substrate. In this U.S. patent, the amorphous silicon contains 1 to 40 atomic % of halogen atoms, which enhances heat resistance and which permits the photoconductive layer of the light receiving member for electrophotography to have good electrical and optical characteristics.
Japanese Patent Application Laid-open No. 57-115556 describes the technology for forming a surface barrier layer of a non-photoconductive, amorphous material containing silicon atoms and carbon atoms on the photoconductive layer of an amorphous material the matrix of which is silicon atoms, in order to improve the electrical, optical, and photoconductive characteristics such as the dark resistance, photosensitivity, and optical response, operating circumstance characteristics such as humidity resistance, and aging resistance.
U.S. Pat. No. 4,788,120 describes the technology using an amorphous material containing components of silicon atoms, carbon atoms, and 41 to 70 atomic % of hydrogen atoms for the surface layer.
Japanese Patent Application Laid-open No. 62-83470 discloses the technology for obtaining high-quality images without ghost phenomenon by keeping characteristic energy of exponential tail of photoabsorption spectrum not more than 0.09 eV in the photoconductive layer of a photosensitive member for electrophotography.
Japanese Patent Application Laid-open No. 58-21257 discloses the technology for obtaining the photosensitive member with high resistance and with a wide photosensitive region by changing the temperature of the substrate during formation of the photoconductive layer, thereby changing the band gap in the photoconductive layer.
Japanese Patent Application Laid-open Nos. 59-143379 and 61-201481 disclose the technology for obtaining a photosensitive member with high dark resistance and with high sensitivity by stacking hydrogenated amorphous silicon layers of different hydrogen contents.
On the other hand, Japanese Patent Application Laid-open No. 60-95551 discloses the technology for, in order to enhance the quality of image of the amorphous silicon photosensitive member, carrying out image forming steps of charging, exposure, development, and transfer while maintaining temperatures near the surface of a photosensitive member at 30.degree. C. to 40.degree. C., thereby preventing decrease of surface resistance due to adsorption of water in the surface of a photosensitive member and thus preventing image smearing occurring therewith.
These techniques improved the electrical characteristics, optical characteristics, photoconductive characteristics, and operating circumstance characteristics of a light receiving member for electrophotography and also improved the quality of image therewith.
The conventional light receiving members for electrophotography having the photoconductive layer made of the amorphous silicon based material, however, were improved in performance in each of the electrical characteristics, optical characteristics, photoconductive characteristics, operating circumstance characteristics, and durability, but they were not sufficient in the total aspect yet and there is still room for further improvement.
Particularly, improvements in the quality of image, copying speed, and durability of electrophotographic apparatus are rapid, while the light receiving member for electrophotography needs to be further improved in the electrical characteristics and photoconductive characteristics and to be greatly enhanced in performance under all circumstances while maintaining the chargeability and sensitivity. As a consequence of accomplishment of improvements in optical exposure device, developing device, transferring device, etc. in the electrophotographic apparatus in order to improve the image characteristics of electrophotographic apparatus, the light receiving member for electrophotography was required to have higher image characteristics than heretofore.
Under the above circumstances the aforementioned conventional techniques enabled one to achieve some improvement in characteristics as to the above problems, but they are not sufficient yet as to further improvements in the chargeability and the quality of image. Especially, for achieving still higher image quality of the amorphous silicon based light receiving member, demands are becoming higher and higher for suppressing variation of electrophotographic characteristics (such as chargeability) due to change in ambient temperature (improvement in operating circumstance characteristics) and for decreasing the optical memory such as blank memory or ghost (improvement in the photoconductive characteristics).
For example, a method employed for preventing the so-called image smearing on the surface of a photosensitive member is to keep the surface temperature of a photosensitive member at about 40.degree. C. by a drum heater mounted in a copier, as described aforementioned Japanese Patent Application Laid-open No. 60-95551. The conventional photosensitive member, however, had strong temperature dependence on chargeability because of generation of pre-exposure carriers or thermally excited carriers. Accordingly, it must be used in a state of lower chargeability during practical operation circumstances in the copier than in its original higher chargeable state. For example, the chargeability was approximately 100 V lower when heated to about 40.degree. C. by the drum heater than during operation at room temperature.
In some cases, the drum heater is powered even during non-operating periods of the copier (for example, during the night), thereby preventing the image smearing caused by adsorption of ozone products, produced by corona discharge of charging device, to the surface of a photosensitive member. However, the supply of power during the night or the like when the copier is not used is avoided as much as possible for power saving currently. If the copying operation is started from the state without supply of power, the temperature will increase gradually around the photosensitive member in the copier and the chargeability will be lowered therewith. This caused a phenomenon that the density of image varied during the copying operation.
Further, continuous and repetitive copying operations with the same original posed problems on the improvement in the quality of image, including a phenomenon that a residual image of image exposure in a preceding copying step remained on an image upon further copying (ghost), a phenomenon that a density difference occurred on a copy image because of blank exposure of irradiation of a photosensitive member between sheets during continuous copying operations in order to save the toner (blank memory), and so on.
On the other hand, with the spread of use of computers to offices and ordinary households in recent years, the electrophotographic apparatus is being required to be digitized in order to function not only as a copier as before, but also as a facsimile machine or as a printer. The majority of semiconductor lasers and LEDs used as an exposure light source for that purpose are those of relatively long wavelengths ranging from near infrared to red visible light in terms of radiation intensity. Therefore, they were required to solve problems on characteristics which have never been raised with conventional analog machines using halogen light.
Especially, with use of the semiconductor lasers or LEDs, characteristics drawing attention were shifting of the relation between exposure value and surface potential of a photosensitive member, so called E-V characteristic (curve), depending upon temperature (temperature characteristic of sensitivity) and degradation of linearity (linearity of sensitivity) of the E-V characteristic (curve). Namely, in digital machines using the semiconductor laser or the LED as an exposure light source, when the temperature of a photosensitive member was not controlled by the drum heater described above, the temperature characteristic of sensitivity and the degradation of linearity of sensitivity posed a new problem that the sensitivity varied depending upon the ambient temperature to change the density of image.
It is, therefore, considered that in designing the light receiving member for electrophotography it is necessary to achieve improvements from all viewpoints including the layer structure, the chemical composition of each layer, and so on and to achieve further improvements in characteristics of the amorphous silicon based materials per se.